Xie Qing, Zhang Yu, Janzen Eli, Edgar James H, Xu Xiaoji G
Department of Chemistry, Lehigh University, Bethlehem, PA, US.
Ames National Laboratory, Iowa State University, Ames, IA, US.
Nat Nanotechnol. 2024 Aug;19(8):1108-1115. doi: 10.1038/s41565-024-01670-w. Epub 2024 May 15.
For decades, infrared (IR) spectroscopy has advanced on two distinct frontiers: enhancing spatial resolution and broadening spectroscopic information. Although atomic force microscopy (AFM)-based IR microscopy overcomes Abbe's diffraction limit and reaches sub-10 nm spatial resolutions, time-domain two-dimensional IR spectroscopy (2DIR) provides insights into molecular structures, mode coupling and energy transfers. Here we bridge the boundary between these two techniques and develop AFM-2DIR nanospectroscopy. Our method offers the spatial precision of AFM in combination with the rich spectroscopic information provided by 2DIR. This approach mechanically detects the sample's photothermal responses to a tip-enhanced femtosecond IR pulse sequence and extracts spatially resolved spectroscopic information via FFTs. In a proof-of-principle experiment, we elucidate the anharmonicity of a carbonyl vibrational mode. Further, leveraging the near-field photons' high momenta from the tip enhancement for phase matching, we photothermally probe hyperbolic phonon polaritons in isotope-enriched hBN. Our measurements unveil an energy transfer between phonon polaritons and phonons, as well as among different polariton modes, possibly aided by scattering at interfaces. The AFM-2DIR nanospectroscopy enables the in situ investigations of vibrational anharmonicity, coupling and energy transfers in heterogeneous materials and nanostructures, especially suitable for unravelling the relaxation process in two-dimensional materials at IR frequencies.
几十年来，红外（IR）光谱学在两个不同的领域取得了进展：提高空间分辨率和拓宽光谱信息。尽管基于原子力显微镜（AFM）的红外显微镜克服了阿贝衍射极限并达到了亚10纳米的空间分辨率，但时域二维红外光谱（2DIR）能够深入了解分子结构、模式耦合和能量转移。在这里，我们弥合了这两种技术之间的界限，开发了AFM-2DIR纳米光谱学。我们的方法结合了AFM的空间精度和2DIR提供的丰富光谱信息。这种方法通过机械检测样品对尖端增强飞秒红外脉冲序列的光热响应，并通过快速傅里叶变换（FFT）提取空间分辨的光谱信息。在原理验证实验中，我们阐明了羰基振动模式的非谐性。此外，利用尖端增强产生的近场光子的高动量进行相位匹配，我们对同位素富集的六方氮化硼（hBN）中的双曲线声子极化激元进行了光热探测。我们的测量揭示了声子极化激元与声子之间以及不同极化激元模式之间的能量转移，这可能是由界面处的散射辅助完成的。AFM-2DIR纳米光谱学能够对异质材料和纳米结构中的振动非谐性、耦合和能量转移进行原位研究，特别适合于揭示二维材料在红外频率下的弛豫过程。
